This is the first time they are meeting their daughters boyfriend, it's a big thing for parents. The thing is though, you are both under their roof and should stick to their rules. I know it's unfair and frustrating but sometimes these things have to be done, just to keep on their good side. If you and your boyfriend both want him to get to know your parents, don't cause any trouble by pushing it too far. By all means explain your situation, try and stand up for yourself and make them realise that you love him, and that you are old enough to sleep in the same bed. I just wouldn't get into a huge argument about it, because it could stop your parents and your boyfriend from getting on as well as they would have done without the hassle of sharing a bed. They have set down their rules, and by pushing them, they could easily put some of that blame on your boyfriend and that's no way to start off the weekend.
Ask them about it, but if they are dead set against it, respect the rules they have put down. To them this is a brand new person, someone that their daughter cares for a lot, they don't know him like you do therefore they are going to have their doubts. You need to let them get to know him before you can really expect certain things from them. Give them a break, they are just doing what they think is best.